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Tue, 23 Sep 2025 11:03:23 -0700 (PDT) Message-ID: <88998fa9-b454-45c1-a8e2-164d2e2d94c0@gmail.com> Date: Tue, 23 Sep 2025 11:03:18 -0700 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] sched/deadline: only set free_cpus for online runqueues From: Doug Berger To: Peter Zijlstra , Juri Lelli Cc: Ingo Molnar , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , linux-kernel@vger.kernel.org, Florian Fainelli References: <20250815012236.4053467-1-opendmb@gmail.com> <20250903075436.GN3245006@noisy.programming.kicks-ass.net> <5bb8ad3a-9d05-4d07-8d4b-207be28cceb1@gmail.com> Content-Language: en-US In-Reply-To: <5bb8ad3a-9d05-4d07-8d4b-207be28cceb1@gmail.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 9/4/2025 9:43 PM, Doug Berger wrote: > On 9/3/2025 12:54 AM, Peter Zijlstra wrote: >> On Mon, Aug 18, 2025 at 02:58:19PM +0200, Juri Lelli wrote: >>> Hello, >>> >>> On 14/08/25 18:22, Doug Berger wrote: >>>> Commit 16b269436b72 ("sched/deadline: Modify cpudl::free_cpus >>>> to reflect rd->online") introduced the cpudl_set/clear_freecpu >>>> functions to allow the cpu_dl::free_cpus mask to be manipulated >>>> by the deadline scheduler class rq_on/offline callbacks so the >>>> mask would also reflect this state. >>>> >>>> Commit 9659e1eeee28 ("sched/deadline: Remove cpu_active_mask >>>> from cpudl_find()") removed the check of the cpu_active_mask to >>>> save some processing on the premise that the cpudl::free_cpus >>>> mask already reflected the runqueue online state. >>>> >>>> Unfortunately, there are cases where it is possible for the >>>> cpudl_clear function to set the free_cpus bit for a CPU when the >>>> deadline runqueue is offline. When this occurs while a CPU is >>>> connected to the default root domain the flag may retain the bad >>>> state after the CPU has been unplugged. Later, a different CPU >>>> that is transitioning through the default root domain may push a >>>> deadline task to the powered down CPU when cpudl_find sees its >>>> free_cpus bit is set. If this happens the task will not have the >>>> opportunity to run. >>>> >>>> One example is outlined here: >>>> https://lore.kernel.org/lkml/20250110233010.2339521-1-opendmb@gmail.com >>>> >>>> Another occurs when the last deadline task is migrated from a >>>> CPU that has an offlined runqueue. The dequeue_task member of >>>> the deadline scheduler class will eventually call cpudl_clear >>>> and set the free_cpus bit for the CPU. >>>> >>>> This commit modifies the cpudl_clear function to be aware of the >>>> online state of the deadline runqueue so that the free_cpus mask >>>> can be updated appropriately. >>>> >>>> It is no longer necessary to manage the mask outside of the >>>> cpudl_set/clear functions so the cpudl_set/clear_freecpu >>>> functions are removed. In addition, since the free_cpus mask is >>>> now only updated under the cpudl lock the code was changed to >>>> use the non-atomic __cpumask functions. >>>> >>>> Signed-off-by: Doug Berger >>>> --- >>> >>> This looks now good to me. >>> >>> Acked-by: Juri Lelli >> >> So I just had a look at said patch because Juri here poked me; and I >> came away with the feeling that cpudl_clear() is now a misnomen, seeing >> how it is called from rq_online_dl(). >> >> Would cpudl_update() be a better name? > Thanks for taking a look. > > I don't really have a dog in any fights over naming here, but it seems > to me that cpudl_clear and cpudl_set are intended to be complementary > functions and the naming reflects that. It would appear that these are > primarily intended to maintain the cpudl max-heap entries which is what > are being set and cleared. > > rq_online_dl() would now call one or the other based on whether any > deadline tasks are running on the queue when it is onlined to ensure > that the max-heap is valid. This either clears a stale entry that may > occur from scenarios like the ones I'm running into or set the entry to > the current deadline. In this context the names seem appropriate to me. > > Renaming cpudl_clear to cpudl_update may be more confusing since the > comment for cpudl_set reads "cpudl_set - update the cpudl max-heap". > > I don't feel that the name change is relevant to my patch, but if we > want to do it concurrently maybe cpudl_clear_max_heap() and > cpudl_set_max_heap() would be more meaningful. > > Please let me know how you would like to proceed, >     Doug Is there any way I can help to move this patch forward? Thanks, Doug